1. Field of the Invention
The present invention relates to a transistor or inverter resistance welding power supply apparatus.
2. Description of the Related Arts
Transistor or inverter resistance welding power supply apparatuses have hitherto often be used for precision resistance welding of materials such as metal foils or small-gage wires.
The transistor power supply is a method substantially allowing only transistors to intervene between a capacitor storing welding energies and welding electrodes without interposition of any welding transformer therebetween, the transistors serving to control a welding current discharged from the capacitor, previous to the supply to the materials to be welded together. In the inverter power supply a commercial-frequency power is converted by an inverter into a high-frequency power which in turn is fed via the transformer to the materials to be welded together. Both the power supply methods are able to flow a DC welding current with rapid rise of the welding current and provide a minute constant-current control or constant-voltage control.
In the transistor or inverter resistance welding power supply apparatus, as described above, the constant-current control and the constant-voltage control are both often employed as welding current control modes. Such a conventional resistance welding power supply apparatus is arranged to make an alternative selection of either of the constant-current control and the constant-voltage control for a single welding current supply. Thus, in case of selection of the constant-current control, a desired current value is set and entered without entry of any voltage set value for the constant-voltage control. On the contrary, in case of selection of the constant-voltage control, a desired voltage value is set and entered without entry of any current set value.
However, even though the desired welding current flows as a result of selection of the constant-current control, too high a voltage applied to the materials to be welded together may bring about any spatters, possibly resulting in defective welding. As opposed to this, even though the desired welding voltage is applied as a result of selection of the constant-voltage control, an excessive welding current may also result in defective welding or may cause faults in the power supply circuit.
In addition, the conventional power supply apparatus has merely provided a feedback control for the purpose of execution of current supply in a given welding current control method during the welding current supply, but has by no means provided a feature to detect any abnormal phenomena associated with parameters out of selection as described above or a feature to ensure a quick transition to proper processing upon the occurrence of any abnormality.